HPLC Purity Explained: What the Percentage Really Means

The principle

High-performance liquid chromatography separates the components of a sample as they travel through a packed column. For peptides, reversed-phase HPLC with UV detection is the most common approach: molecules elute at different times depending on how strongly they interact with the column.

Reading the chromatogram

The output is a chromatogram: a trace with peaks over time. The main peak corresponds to the target peptide; smaller peaks are related substances such as truncated or modified sequences. Purity is typically calculated as the area of the main peak divided by the total area of all integrated peaks.

Limits of a single number

A purity percentage depends on the method: wavelength, column, gradient and integration settings. Two laboratories can report slightly different figures for the same material. Impurities that co-elute with the main peak or do not absorb at the chosen wavelength are not captured.

This is why purity is best read together with identity testing (for example mass spectrometry) and with the batch number, rather than as a stand-alone claim.